Hydroxyquinoline–Calix[4]arene Conjugates as Ligands for Polynuclear Lanthanide Complexes: Preparation, Characterization, and Properties of a Dinuclear EuIIIComplex

Article abstract of DOI:10.1002/ejic.201601326

The synthesis of calixarene-based N₄O₄-donor ligand H₆L, in which two 8-hydroxyquinoline-2-carbaldehyde units are appended via hydrazone-carbonylmethoxy linkages in a 1,3 arrangement to the lower rim of tert-butylcalix[4]arene, and its coordination properties towards Eu^IIIare described. The reaction of H₆L with Eu(NO₃)₃·6H₂O produced dinuclear complex [Eu₂(H₃L)₂(MeCN)₂] (4). The Eu^IIIions in 4 are well separated from each other (d > 8 ?) and each is coordinated by two tetradentate hydroxyquinoline-2-carbaldehyde hydrazone substituents from two triply deprotonated (H₃L)^3–ligands in a dodecahedral geometry. As a consequence there are no exchange interactions between the Eu^IIIions, and the magnetic properties are solely associated with thermal population of excited (⁷F₁–⁷F₆) terms. The spin–orbit coupling constant λ is 325.2(4) cm^–1. Moreover, H₆L was found to extract Eu^IIIand Tb^IIIfrom aqueous solution at pH 7–8, as established by radiotracing with¹⁶⁰Tb and¹⁵²Eu.

Full text of DOI:10.1002/ejic.201601326

A Journal of
Accepted Article
Title: Hydroxyquinoline-Calix[4]arene-Conjugates as Ligands
for Polynuclear Lanthanide Complexes: Preparation,
Characterization, and Properties of a Dinuclear Eu(III) complex
Authors: Berthold Kersting, Astrid Jäschke, Markus Kischel, and
Alexander Mansel
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601326
Link to VoR: http://dx.doi.org/10.1002/ejic.201601326

10.1002/ejic.201601326
European Journal of Inorganic Chemistry
1
Hydroxyquinoline-Calix[4]arene-Conjugates as Ligands for Polynuclear
Lanthanide Complexes: Preparation, Characterization, and Properties of a
Dinuclear Eu(III) complex
Astrid Jäschke,[a] Marcus Kischel,[a] Alexander Mansel,[b] and Berthold Kersting,*[a]
aInstitut für Anorganische Chemie, Universität Leipzig, Johannisallee 29, D-04103 Leipzig
(Germany), Fax: +49/(0)341-97-36199 E-Mail: b.kersting@uni-leipzig.de
bHelmholtz-Zentrum Dresden-Rossendorf; Institute of Resource Ecology; Reactive Transport
Division; Research Site Leipzig; http://www.hzdr.de; Permoserstrasse 15; 04318 Leipzig
Abstract: The synthesis of a calixarene-based N4O4 donor ligand H6L, in which two 8-
hydroxyquinoline-2-carbaldehyde units are appended via hydrazone-carbonylmethoxy
linkages in a 1,3-arrangement to the lower rim of tert-butyl-calix[4]arene, and its coordination
.
properties towards Eu(III) are described. The reaction of H6L with Eu(NO3)3 6H2O produces a
dinuclear complex [Eu2(H3L)2(MeCN)2] (4). The Eu(III) ions in 4 are well-separated from
each other (d > 8 Å) each being coordinated by two tetradentate hydroxyquinoline-2-
3

carbaldehyde-hydrazone substituents from two triply deprotonated molecules (H3L) in a
dodecahedral geometry. As a consequence there are no exchange interactions between the
Eu(III) ions, and the magnetic properties are solely associated with thermal population of
1
excited (7F1-7F6) terms. The spin-orbit coupling constant  is 325.2(4) cm . Moreover, the
ligand H6L was found to extract Eu(III) and Tb(III) at pH 7-8 from aqueous solution, as
established by radiotracing using the short-lived radio nuclide 160Tb and 152Eu.
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European Journal of Inorganic Chemistry
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Keywords: Polynuclear Complex / 8-Hydroxyquinoline / Calix[4]arene / Europium /
Extraction
Introduction
The 8-hydroxyquinoline ligand (HOQ) is one of the most popular gravimetric reagents for
trivalent metal ions because of the low solubility of the resulting complexes.[1] 8-
hydroxyquinolinolato complexes of the rare earth elements have also been studied as
extraction agents,[2] infrared-emitting molecular materials,[3–5] and as therapeutic agents for
metal chelation,[6] while some derivatives of 8-hydroxyquinoline were employed in the
construction of self-assembled lanthanide helicates[7] and heterobimetallic lanthanide/alkaline
earth complexes.[8] However, only a few rare earth quinolinolato complexes have been
obtained as single-crystals suitable for X-ray diffraction analysis due to their low solubility.
So far four types of structures were encountered: mononuclear Ln(OQ)3 or Ln(OQ)4
[3]
[4]

complexes, trinuclear Ln3(OQ)8(L') compounds (e.g. Ln = Yb, L' = NO3 ; Ln = Yb, L' =
[9]
CH3CO2 ) and trinuclear Ln3(OQ)9 species (Ln = Er,[10] Ho,[11] Yb,[12] Dy[13]). Given the
unusual magnetic properties of polynuclear lanthanide complexes,[14-16] it is remarkable that
the magnetic properties of only a few hydroxyquinolinolato complexes have been
studied.[13,17]

A few derivatives of the parent 8-hydroxyquinolinolato ligand were designed to increase
the solubility and to extend the coordination properties of this versatile ligand system.[18]
Bünzli has introduced carbamide units in the 2-position to generate a tridentate ligand.[19]
Albrecht has reported a semicarbazone derivative of 8-hydroxyquinoline-2-carbaldehyde,[20]
which was found to be an effective tetradentate ligand for rare earth metals. The structures of
mononuclear (Y, Er, Ho) and dinuclear complexes (La, Eu) were reported. The same group
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
3
also reported a ditopic hydroxquinolinolato ligand designed for dinuclear lanthanide
helicates.[21]
As part of a program aimed at producing and characterizing discrete polynuclear
lanthanide complexes, we became interested in the coordination chemistry of a tert-butyl-
calix[4]arene-based 8-hydroxyquinolinolato ligand H6L (see scheme 1).
Our motivation for appending the hydroxyquinolinolato units to a calix[4]arene backbone
were as follows: 1) The calixarenes are easily accessible and can be tailored synthetically by
altering the moieties grafted on the lower rim.[22] 2) Several ligand systems utilizing
calix[4]arene backbones have been reported and their complexation properties were
investigated.[23–26] 3) The calix[4]arenes often confer solubility on the compounds,[27] and
their coordination chemistries are more specific (and controllable) than their unmodified
parents.[28–30] 4) There are only a few examples in the literature in which quinoline or 8-
hydroxyquinoline units are tethered on a calix[4]arene backbone,[31,32] and none of these have
been structurally characterized.[33] 5) The macrocyclic platform can be used for
preorganization of multicoordinate and chelating groups in two-phase solvent extractions for
the separation of rare earth elements.[34–38]
Recently, we have reported on the synthesis and coordination properties of some 1,3-
functionalized calix[4]arene carboxylate ligands.[39] Herein we report on the synthesis,
preparation and molecular structure of the ligand H6L and demonstrate its ability to support a
discrete dinuclear europium complex. Thus, the syntheses, crystal structures and magnetic
properties of the homoleptic complex [Eu2(H3L)2(MeCN)2] (4) are described. The properties
of H6L in the extraction of Eu(III) and Tb(III) from aqueous solution are also described.
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10.1002/ejic.201601326
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Results and Discussion
Synthesis and characterization of the ligand H6L
The new ligand H6L was prepared in three steps, starting from para-tert-butylcalix[4]arene,
[22] as depicted in Scheme 1.
Scheme 1. Synthesis of H6L.
The reaction of 1 with ethyl bromoacetate in acetonitrile gave calix[4]arene 2, with the two
ethyl acetate groups appended in 1,3-position of the calix[4]arene backbone. [40] Treatment of
2 with hydrazine monohydrate gave the corresponding hydrazide 3 in nearly quantitative
yield. [41] In the last step, 8-hydroxyquinoline-2-carbaldehyde was subjected to a condensation
reaction with the hydrazide 3 to give the target compound, which was isolated in the neutral
H6L form. The yield was 73 % (based on 3). H6L is readily soluble in chloroform and
dichloromethane, less soluble in acetonitrile, and practically insoluble in ethanol and
methanol. All compounds gave satisfactory elemental analysis and their formulation was
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
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confirmed by ESI-MS, 1H, 13C NMR, IR, and UV/vis spectroscopy. H6L was also
characterized by X-ray crystallography. Table 1 lists selected spectroscopic data.
The cone conformation of the macrocyle H6L was confirmed by 1H NMR spectroscopy
(see Figure S1, Supporting Information), by the characteristic doublets at 4.3 and 3.6 ppm for
the diastereotopic methylene protons.[42] The regioselective distal 1,3-substitution lowers the
symmetry of the calix[4]arene to C2, to cause a splitting of the individual NMR signals. Thus,
two singlets of equal intensity are observed for the tert-butyl groups and the aromatic protons
of the calix[4]arene unit. The signal at 11.96 ppm is most-likely associated with the hydroxyl
group of the 8-hydroxyquinoline unit. This OH group resonates at a quite low field, indicating
that it is involved in a hydrogen bonding interaction. For comparison, the hydroxyl H atoms
of the calix[4]arene skeleton are observed at 8.38 ppm. The 13C NMR spectrum of H6L shows
one signal for the methylene carbon atoms at 32.8 ppm which is also in agreement with the
cone conformation.[43] One signal for the imine linkages (152.6 ppm) and one signal for the
carbonyl group (163.8 ppm) agree with a C2 symmetric structure.
Table 1: Selected spectroscopic data for H6L and the dinuclear complex 4.
H6L
4
1 a)
IR, ν[cm ]
1697 (C=O)
1597 (C=N)
1601 (C=O)
1588 (C=N)
1192 (CO
1106 (CO)
UV/vis,
200 (12892)
283 (7935)
320 (2678)
203 (13710)
290 (4325)
350 (2047)
λmax [nm]
1
1 b)
(ε, M cm )
ESI-MS,
m/z c)
[M+H]+
1103.6
[EuH4L(H2O)5]+
1343.5
a) ATR spectra (ZnSe crystal). b) UV/vis spectra were recorded in MeCN solution at ambient
temperature; c) ESI(+) in CH3CN.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
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.
Description of the crystal structure of H6L.(CD2Cl2)4 H2O
.
Single crystals of H6L.(CD2Cl2)4 H2O suitable for X-ray diffraction analysis could be
obtained from CD2Cl2. The asymmetric unit comprises one molecule of the neutral ligand
H6L and dichloromethane and water solvate molecules. Figure 1 shows the molecular
structure of H6L. Selected bond lengths and angles are given in the figure caption. The
calix[4]arene unit adopts a flattened cone conformation, which is stabilized by intramolecular
OH…O hydrogen bonds between the phenol H and the ether O atoms. The distortion from the
ideal cone conformation is clearly evident from the different folding angles between the best
plane fitted to the four methylene carbon atoms and the planes through the four aromatic rings
of the calix[4]arene skeleton (ring A: 69.5°, ring B: 45.0°, ring C: 70.3°, ring D: 45.6°).
One dichloromethane molecule is included in the cavity held in place by CH…π interactions
with the aromatic rings (d(C…centroid) = 3.823(1) Å and 3.628(1) Å), as observed in other
calixarene host-guest complexes.[44] The remaining CD2Cl2 molecules fill up the interstitial
space between molecules. There are several intramolecular and intermolecular OH…O and
NH…O hydrogen bonding interactions between the lower rim of the calix[4]arene and the
dangling hydrazone units as illustrated in Figure 1. Note that the amide group of one
substituent adopts the rare cis configuration, presumably stabilized by the bifurcated
O2H…O5 hydrogen bond. The other amide group, on the other hand, adopts the trans
configuration, with a weaker N4H…O2 hydrogen bond of length 3.247 Å.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
7
.
Figure 1. Molecular structure of H6L.(CD2Cl2)4 H2O. Thermal ellipsoids are drawn at 50%
probability level. Solvate molecules and H atoms, expect for those bonded to N and O,
omitted for clarity. Only one orientation of a disordered tert-butyl group is displayed. Selected
bond lengths / Å and angles / °: O1…O2 2.729(4), O3…O4 2.753(1), O5…O2 3.047(2), O2…N4
3.247(2), N1…O7ꞌ 2.963(3); O2-H2…O1 153.2(8), O4-H4…O3 163.4(2), O2-H2…O5 128.8(2),
N4-H70…O2 171.7(2), N1-H71…O7ꞌ 166.5(8). Symmetry code: 1x, 1y, 1z (ꞌ).
The stacking interactions between the hydroxyquinolinecarbaldehyde-carbonylhydrazone
units are also worth mentioning. Two different arrangements can be distinguished (Figure 2).
Thus, the hydrazone unit bonded to O1, which has the amide group in the cis configuration, is
involved in charge transfer interactions with the e-rich hydroxyquinoline heterocycle of an
adjacent molecule, as manifested in the short centroid(X1ꞌ)…N1 and centroid(X2ꞌ) …C47
distances of 3.313(2) and 3.160(1) Å, respectively. As a consequence the hydroxyquinoline N
and OH atoms engage in intra- rather than in intermolecular hydrogen bonding interactions.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
8
.
Figure 2. Stacking interactions between the substituents in crystals of H6L.(CD2Cl2)4 H2O.
Thermal ellipsoids are drawn at 50% probability level. Calix[4]arene units omitted for clarity.
The other hydrazone unit bonded to O3 is not involved in  stacking interactions with
quinoline units. The parallel arrangement is in this case brought about by CH–
interactions,[45] with a corresponding centroid(X3ꞌ)…C57 distance at 3.325(3) Å. The presence
of orientation b may be traced to the trans configuration of the amide bond, which brings the
methylene group into the vicinity of the hydroxyquinolinesꞌ phenyl ring.
Synthesis and characterization of the dinuclear Eu(III) complex [Eu2(H3L)2(MeCN)2 (4)
When the macrocycle H6L is treated with 1 eq. of Eu(NO3)3·6H2O in acetonitrile in the
presence of NEt3 a red solution forms, from which a dark red product of composition
[Eu2(H3L)2(MeCN)2] (4) can be isolated in 38 % yield (Scheme 2). Interestingly, the nitrate
ions are not incorporated as coligands, suggesting that the coordination sites of the Eu(III)
ions are saturated by the donor atoms of H6L alone. This is in contrast to the behavior of
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
9
related lanthanide hydroxyquinolinolate complexes, in which additional coligands are bound
to the lanthanide(III) ions.[18] A typical example is [LnLꞌ(NO3)(DMF)2] (where Lꞌ is a Schiff
base derived from 8-hydroxyquinoline carbaldehyde and aroylhydrazides), which binds
[18c]

additional NO3 and DMF molecules.
Scheme 2. Synthesis of [Eu2(H3L)2(MeCN)2] (4).
Complex 4 gave satisfactory elemental analyses and was fully characterized by
spectroscopic methods (MS, UV/Vis, IR) and X-ray crystallography. The electrospray
ionization mass spectrum (ESI(+)-MS) did not reveal a molecular ion peak for a [Eu2(H3L)2]
species, only a peak for a [Eu(H4L)(H2O)5]+ cation at m/z = 1343.5 was detected (Table 1). A
comparison of the ATR infrared spectrum showed clear evidence for coordination of H6L
(Figure S3 and S4). Marked shifts for the ν(C=O), ν (C=N) and ν(C–O) bands from 1697,
1
1
1597 and 1192 cm in H6L to 1601, 1588 and 1106 cm in 4, indicate the involvement of the
carbonyl group, the aromatic N atom as well as the hydroxy group of the quinoline unit in the
binding process. The coordination of the quinoline unit could also be inferred by UV/vis
spectroscopy. The UV/vis spectrum of H6L in acetonitrile (Figure S7) displays three
absorption bands at 203 nm, 290 nm, and 350 nm, attributable to nπ* and ππ* transitions
(Table 1). Upon coordination of Eu(III), the ππ* transition at 350 nm is red-shifted by 30
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
10
nm relative to the corresponding transition in H6L. Further conclusions regarding the exact
3

binding mode of (H3L) could not be drawn from the spectroscopic data. On these grounds
an X-ray crystallographic study of [Eu2(H3L)2] was undertaken.
Description of the crystal structure of [Eu2(H3L)2(MeCN)2](MeCN)x (4·(MeCN)x) (x ~
21)
Red crystals of 4·(MeCN)x suitable for X-ray crystallographic analysis were obtained by
.
slow evaporation of an acetonitrile solution containing H6L and Eu(NO3)3 6H2O in a 1:1
molar ratio. Crystals of 4·(MeCN)x are monoclinic, space group P21/n. The asymmetric unit
comprises one molecule of the neutral [Eu2(H3L)2(MeCN)2] complex 4 and approximately 21
MeCN solvate molecules. Two MeCN are situated in the calixarene cages, as observed in
many other calix[4]arene structures,[46] the other MeCN molecules (which are readily lost
upon standing in air) fill the interstitial space between the molecules and were found to be
heavily disordered. Figure 3 shows ORTEP and van der Waals diagrams of the neutral
complex. Table S2 lists selected bond lengths and angles. The two Eu(III) ions are well-
separated from each other (Eu1–Eu2 8.343(2) Å) being coordinated by two tetradentate 8-
hydroxyquinoline-2-carbaldehyde-hydrazone substituents from two calix[4]arene units.
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10.1002/ejic.201601326
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11
Figure 3. ORTEP (left) and space filling (right) representation of the molecular structure of
the neutral complex [Eu2(H3L)2(MeCN)2] in crystals of 4·(MeCN)x. Thermal ellipsoids are
drawn at the 30% probability level. Color code: C grey, N blue, O red, Eu green. In the
ORTEP representation, all hydrogen atoms are omitted for clarity. Default values for van der
Waals radii as implemented in ORTEP 3 for Windows were used for the space filling
representation.
3

Charge considerations imply the presence of a triply deprotonated form (H3L) of H6L (2
× RO (quinolinolate), 1 × =NN=CRO), because counterions are absent and the +3
oxidation state of the europium atoms is unambiguous in this case. The presence of one
deprotonated ligand arm with a =NN=CRO valence structure as opposed to that of the
protonated =NNHCR=O arm is also implied by the different N=C and CO bond distances
(1.295(4) Å (C58A=N4A) and 1.345(4) Å (C58AO7) for the N=CRO versus 1.334(1) Å
(C46N1) and 1.285(3) Å (C46=O5) for the NHCR=O moiety (Figure 4). The bond
distances for the protonated ligand arm compare quite well with the corresponding distances
in [Ce(PBH)2(NO3)(NCS)(H2O)] (PBH = 2-pyridinecarboxaldehyde-benzoylhydrazone),
where the benzoylhydrazone unit is bound in its protonated form.[47] The presence of a
protonated and a deprotonated hydrazone unit is also reflected in the EuN and EuO
distances. For example, the EuN2 and EuO5 distances involving the deprotonated ligand
arm are significantly shorter than the corresponding Eu1N5A and Eu1O7A distances with
the protonated arm, and the same holds for the other Eu site (Table S2). Unlike in H6L, all
amide functions adopt a trans configuration.
The quinoline units coordinate the Eu(III) ions in a nearly perfect dodecahedral fashion
(Figure 4), the angle between the planes through the trapezoids spanned by the donor atoms
being 88.9° (O6N3N2O5/O8AN6AN5AO7A) and 88.5°, respectively
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10.1002/ejic.201601326
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12
(O8N6N5O7/O5AN2AN3AO6A, not shown in Figure 5).[48] The average EuO and EuN
distances amount to 2.353(4) Å and 2.523(5) Å, respectively. These bond lengths are much
smaller than the EuO (2.487(3) Å) and EuN (2.717(4) Å) average distances reported for the
complex [LEu(NO3)2.5(DMF)] (L = 2-[(8-hydroxyquinolinyl)methylene]hydrazine-
carboxamide,[19] with a coordination number of 11, but compare well with those of other 8-
coordinate {EuN4O4} complexes.[49]
Upon coordination of the europium ions the calix[4]arene backbone changes its
conformation from a flattened cone (free ligand H6L) to a cone conformation as indicated by
the angles between the best fitted plane through the bridging methylene groups (C2, C8, C14,
C20) and the phenol units of the calix[4]arene (59.3, 63.7, 57.1, 64.4). Intramolecular
hydrogen bonds between the phenols and the oxygen atoms of adjacent ether groups on the
lower rim stabilize again the cone conformation. Additionally, weak hydrogen bonds were
formed between the phenols of the first calix[4]arene and the nitrogen atoms of the quinoline
unit of the second calix[4]arene. Interestingly, π–π stacking interactions between the phenol
units of the macrocycle and the quinoline unit stabilize the closed packing of the structure (see
Figure 3). The centroid…centroid distances between these aromatic units range from 3.30 to
3.41 Å.
Figure 4. Perspective view of the coordination unit in 4 showing the dodecahedral
coordination geometry of the Eu(III) ion. Color code: C grey, N blue, O red, Eu green. Bond
lengths and angles are collected in Table S2.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
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Magnetic Properties
A desolvated sample of complex 4 was further studied by temperature-dependent magnetic
susceptibility measurements using a SQUID-Magnetometer (MPMS Quantum Design) in
applied magnetic fields of 1, 0.5 and 0.1 T in the temperature range 2-300 K (see Figure S8).
Figure 5 displays the temperature dependence of the magnetic susceptibility in the form of a
mT versus T plot in applied magnetic field of 1 T. The mT value at room temperature is
1
found to be 2.77 cm3 K mol , a value which is close to that expected for two non-interacting
1
Eu(III) ions (2.65 cm3 K mol ), whose magnetic properties are associated with thermal
population of the excited terms (7F1 – 7F6). Upon cooling, the mT values decrease steadily,
1
reaching 0.03 cm3 K mol at 2 K. The residual magnetism is due principally to the second
order Zeeman effect[50] in the ground 7F0 multiplet.[51]
Figure 5. mT versus temperature for [Eu2(H3L)2(MeCN)2] (4) in an applied magnetic field of
1.0 T. The solid line is a fit of the experimental data to eq. 1 using the parameter given in the
text.
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10.1002/ejic.201601326
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The 4f electrons of Eu(III) ions are generally shielded by the outer sphere s and p
electrons, and superexchange interactions between the spins of the 4f electrons are usually
very weak. In 4, the Eu(III) ions are well-separated, and the magnetic susceptibility of the
[Eu2(H3L)2(MeCN)2] complex can be fitted with a single-ion Eu(III) model, which only
considers the spin-orbital coupling of Eu(III) ions. The magnetic susceptibility of an Eu(III)
ion for the two-level system (7F1, 7F0) is given by eq. 1, where  is the multiplet spacing, Na
Avogadros number, B the Bohr magneton and kB the Boltzmann constant.






24kBT
9
2
k T
 
kBT





B




3

exp

NaB2
3kBT





2


m 

(1)


 
kBT










1 3exp


As can be seen from Figure 5, a good fit was possible over the whole temperature range,
1
yielding  = 325.2(4) cm . Since the multiplet spacing is on the order of kT, the first excited
state is thermally populated at 300 K, and the magnetic properties deviate significantly from
the Curie law. The λ parameter agrees with literature values.[52]
Liquid-liquid extractions
In orienting experiments, we studied the ability of H6L to act as an extraction agent for
lanthanide ions, in view of literature reports that functionalized calixarene ligands exhibit
good extraction efficiency.[35,53–57] The two-phase solvent extraction studies were performed
in chloroform/water mixtures and were monitored by radiotracing utilizing 160Tb as well as
152Eu. Figure 6 shows the extraction yields (%E) of H6L towards Tb(III) and Eu(III) as a
function of pH. As can be seen, the extraction yields depend markedly upon pH value,
showing a sigmoidal profile with the lanthanide extraction (%E ~(9598)%) being essentially
nearly quantitative above pH ~7 at a metal to ligand ratio of 1:10. The strong pH dependence
is in good agreement with the acid-base behavior of H6L, which is considered to be present in
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
15
its triply deprotonated form based on the pKa values of the hydroxyquinoline and hydrazone
units, as also implied by the crystal structure of 4 described above. Deprotonation of H6L was
also implied by a shift of the pH value of the aqueous phase that accompanied the extraction
of the Ln(III) ions.
Figure 6. Extraction yields obtained for H6L at different pH values. Conditions:
4
3
chloroform/water (1/1, v/v), [TbIII / EuIII]: 1 × 10 mol/L, [H6L]: 10 mol/l, 25 °C,
equilibration time: 60 min.
Finally, the addition of H2SO4 lead to 92 % back-extraction of the lanthanide ion at a pH
value of 4. Overall, the calix[4]arene-based hydroxyquinoline-ligand H6L described in this
study exhibits good extraction ability for Tb(III) and Eu(III) ions which, due to its solubility
properties, may be of use as an alternative extraction agent for lanthanide ions.
Conclusion
A short and efficient synthesis of the calixarene based ligand H6L with two appended
hydroxyquinoline-2-carbaldehyde-hydrazone groups has been achieved. The ligand was found
to be an effective supporting ligand for polynuclear lanthanide complexes. The reaction of
.
H6L with Eu(NO3)3 6H2O produces a discrete, homoleptic [Eu2(H3L)2(MeCN)2] complex (4),
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
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3

where H6L is in a triply deprotonated (H3L) form with one singly and one doubly
deprotonated ligand arm. The Eu(III) ions in 4 are well-separated from each other (d > 8 Å)
each being coordinated by two tetradentate hydroxyquinoline-2-carbaldehyde-hydrazone
3

substituents from two triply deprotonated molecules (H3L) in a dodecahedral geometry. The
structures of H6L as well as 4 appear to be also governed by inter- and intramolecular CH2–
and – interactions involving the heterocyclic 8-hydroxyquinoline ring. The ligand H6L was
found to extract Tb(III) and Eu(III) at pH 7–8 from aqueous solution, as established by
radiotracing. In addition back-extraction of the lanthanide ions at a pH value of 4 was also
successfully carried out.
Acknowledgement
We sincerely thank the Bundesministerium für Bildung und Forschung (SE-FLECX,
033R132E) and BuildMoNa graduate school for funding of this work. We are thankful to
Prof. Dr. H. Krautscheid and coworkers for providing facilities for X-ray crystallographic
measurements, and to Dr. Karsten Franke and Stefan Gruhne (HZDR, Research Site Leipzig)
for support at the cyclotron facility.
Experimental Section
Materials and Methods. All preparations were carried out in air without specific precautions.
Reagent grade solvents were used throughout. Compounds 1-3 were synthesized according to
known procedures.[21,32,33] All other reagents were obtained from standard commercial sources
and were used without further purifications. Elemental analyses were carried out with a
VARIO EL elemental analyzer. Infrared spectra were recorded on a Bruker VECTOR 22 FT-
IR spectrophotometer. The UV/vis absorption spectra were recorded on a Jasco V-570
UV/vis/NIR spectrometer. NMR experiments were conducted on a BRUKER AVANCE DRX
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
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400. ESI-mass-spectra were recorded on a APEX II FT-ICR device (BRUKER).
Temperature-dependent (350 K - 2 K) magnetic susceptibility measurements were conducted
on polycrystalline samples on a MPMS 7XL SQUID (QUANTUM DESIGN) in external
magnetic fields of 0.1, 0.5 and 1T. Diamagnetic corrections were applied for the sample
holder (gelatine capsule) and the core diamagnetism from the sample (estimated with Pascals
constants).
5,11,17,23-Tetra-p-tert-butyl-25,27-bis[(8-hydroxyquinolinecarbaldehyde-hydrazone-
carbonylmethoxy)]-26,28-dihydroxycalix[4]arene (H6L). Hydrazide 3 (0.5 g; 631 µmol)
was dissolved in 30 mL ethanol. A solution of 8-hydroxyquinoline-2-carbaldehyde (219 mg;
1.26 mmol) in 5 mL ethanol was added dropwise. The yellow solution was refluxed over
night. The obtained pale yellow precipitate was filtrated and washed with ethanol and
acetonitrile. Yield: 521 mg (75 %). 1H NMR (400 MHz, CD2Cl2) cone conformation δ [ppm]
= 11.96 (s, 2H, OH), 8.61 (s, 2H, N=CH), 8.38 (s, 2H, OH), 8.18 (d, 3JHH = 8.8 Hz, 2H), 8.04
(d, 3JHH = 8.7 Hz, 2H), 7.50 (d, 3JHH = 7.9, 2H), 7.39 (dd, 3JHH = 8.2, 1.3, 2H), 7.22 (d, 3JHH
=
8.0 Hz, 2H) (all ArH substituent), 7.2 (s, 4H), 6.99 (s, 4H) (all ArH calixarene), 4.84 (s, 4H,
OCH2); 4.31 (d, 2JHH = 13.5 Hz, 4H, ArCH2Ar), 3.65 (d, 2JHH = 13.5 Hz, 4H, ArCH2Ar), 1.31
(s, 18H, tBu), 1.06 (s, 18H, tBu). 13C{1H} NMR (101 MHz, CD2Cl2) cone conformation δ
[ppm] = 164.36 (C=O); 152.78 (N=CH), 149.75, 149.66, 149.09, 148.14, 144.28, 136.48,
128.91, 118.87, 117.94, 110.75 (ArC substituents), 150.81, 149.75, 132.03, 129.65, 128.65,
126.94, 126.80, 126.28 (all ArC calixarene), 74.69 (OCH2), 33.84, 33,81 (2 × C(CH3)3), 32.80
(ArCH2Ar), 31.20, 30.80 (2 × C(CH3)3). m/z (ESI+, CH3CN): C68H75N6O8 (1103.37) [M+H+]+
1
calc: [M+H+]+: 1103.6, found: 1103.6. IR (ATR): ṽ [cm ] = 3246 ν(NH), 2958 ν(CH), 1697
1
1
ν(CO), 1597 ν(CN), 1192 ν (CO). UV/Vis (MeCN): λmax [nm] (ε[L·mol ·cm ]) = 200
(12892), 283 (7935), 320 (2678). C68H74N6O8 (1102.4): calcd. C 74.02, H 6.76, N 7.62, found
C 74.10, H 6.81, N 7.61.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
18
[Eu2(H3L)2(MeCN)2](MeCN)x (4(MeCN)x, x~21). H6L (75 mg; 68 µmol) was dissolved in
20 mL acetonitrile and NEt3 (19 µL, 136 µmol). A solution of Eu(NO3)3·6H2O (30 mg, 68
mol) in acetonitrile was added. After 2 days red crystals were obtained, which were filtered,
washed with acetonitrile, and dried in air. Crystals of 4(MeCN)x (x ~ 21) quickly lose the ~ 21
MeCN solvate molecules upon standing in air and turn dull. Yield: 65 mg (38 %). m/z (ESI+,
CH3CN): C68H81N6O13Eu (1343.51) [Eu(H3L)(H2O)5]+ calcd: 1343.51, found: 1343.5. IR
~
1
(ATR):

[cm ] = 3358 ν(NH), 2954 ν(CH), 1601 ν(CO), 1588 ν(CN). UV/Vis (MeCN): λmax
1
1
[nm] (ε[L·mol ·cm ]) = 203 (13710), 290 (4325), 344 (2047). C136H142Eu2N12O16 + 2MeCN
(2504.4 + 82.1): calc. C 65.01, H 5.77, N 7.85, found C 65.10; H 5.82; N 7.91.
Crystal structure determinations. The intensity data were collected on a STOE-IPDS-2T-
diffractometer. Graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) was used
throughout. The data were processed with X-AREA and corrected for absorption using STOE
X-Red32.[58] The structures were solved by direct methods (SHELXS-2013)[59] and refined by
full-matrix least-squares on F2. Hydrogen atoms were included on idealized positions;
nonhydrogen atom positions were refined with anisotropic thermal parameters, except some
disordered tert-butyl groups. Figures were generated with Diamond 3.2i, Ortep-III and POV-
Ray v3.7.[60-62] In the crystal structure of H6L.(CD2Cl2)4(H2O), no hydrogen atoms were
calculated for the H2O solvate, which is located in the vicinity of an inversion center. Most
solvate molecules in the structure of 4 were found to be severely disordered and/or partially
occupied. Electron density attributed to heavily disordered acetonitrile molecules was
removed from the structures (and the corresponding F0) with the SQUEEZE algorithm
implemented in the PLATON program suite.[63] The SQUEEZE routine revealed total potential
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
19
solvent area of 5111.9 Å3 (16569.0 Å3 per unit cell, 30.9%) in 4 corresponding to
approximately 21 MeCN molecules per formula unit.64
Liquid-liquid extractions. 160Tb (T1/2 = 72.3 d, Eγ = 879.4 keV, Iγ = 30.1%) and 152Eu (T1/2 =
13.5 a, Eγ = 344.3 keV, Iγ = 26.6%) were purchased from POLATOM (Poland) with a specific
activity of 979.4 MBq/mg 160Tb and 586.9 MBq/mg 152Eu and were dissolved in 1 mL 10
mmol/L nitric acid to give the stock solution. All radiometric measurements (-counting) were
performed by using a Wizard Gamma-Counter 1480 (PerkinElmer) which was calibrated with
an 152Eu-standard (PTB 443-88, PTB, Germany) in the energy range from 50 to 1500 keV.
The ligand H6L was dissolved in chloroform to give solutions with a concentration range from
(0.5 to 1000) µmol/L. The radionuclide stock solutions were diluted to give a radioactivity of
~10 kBq/mL. Additionally, non-radioactive terbium nitrate was added as carrier to give
aqueous solutions from (0.1 to 100) µmol/L. The pH was adjusted to the desired value with
0.1 M nitric acid or ammonia. 3 mL of the organic solution was shaken with 3 mL of the
1
aqueous solution in a stoppered vial on a horizontal shaker at 300 min . The phases were
allowed to equilibrate for 60 min. After separation of the phases for 1 min, an aliquot of 2 mL
of the aqueous phase were measured by -counting in comparison to 2 mL of the aqueous
solution before extraction. The percentage extraction (%E) was calculated by means of the
two activities (Ainitial = radionuclide in aqueous solution before extraction, Afinal = radionuclide
in aqueous solution after extraction) using equation 2. All activities were decaycorrected to
the first measured sample.
[Ainitial  Afinal
]
%E 100
(2)
[A
]
initial
Supporting Information
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
20
Supporting Information contain the X-ray crystallographic format in CIF format, NMR, ATR-
IR, ESI-MS, and UV/Vis spectra for all new compounds. CCDC-1474404 (for H6L) and -
1409111 (for 4.(MeCN)x) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac/data_request/dif.
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changes (which one might expect in view of the de-occupation of ca. 30% of the
original cell volume). The corresponding IR spectra, however, reveal only minor shifts
for the ν(C=O), ν (C=N) and ν(C–O) bands. This indicates the involvement of the
carbonyl group, the aromatic N atom as well as the hydroxy group of the ligands
quinoline units in the coordination of the EuIII ions, and suggests that the integrity of
the [Eu2(H3L)2(MeCN)2] complex is maintained upon desolvation.
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10.1002/ejic.201601326
European Journal of Inorganic Chemistry
27
For the Table of Contents
The synthesis of a new hybrid hydroxyquinoline-calixarene ligand (H6L) has been achieved.
H6L is an effective supporting ligand for polynuclear lanthanide complexes. The neutral,
homoleptic [Eu2L2] complex features two well-separated Eu(III) ions (d > 8 Å) each being
coordinated by two tetradentate hydroxyquinoline-2-carbaldehyde-hydrazone substituents in a
dodecahedral geometry. H6L extracts Eu(III) and Tb(III) from aqueous solution, as
established by radiotracing.
Keywords: Polynuclear Complex / 8-Hydroxyquinoline / Calix[4]arene / Europium /
Extraction
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